High temperature silicon surface providing high selectivity in an oxide etch process

ABSTRACT

A plasma process for etching oxide and having a high selectivity to silicon including flowing into a plasma reaction chamber a fluorine-containing etching gas and maintaining a temperature of an exposed silicon surface within said chamber at a temperature of between 200° C. and 300° C. An example of the etching gas includes SiF 4  and a fluorocarbon gas. The plasma may be generated by a capacitive discharge type plasma generator or by an electromagnetically coupled plasma generator, such as an inductively coupled plasma generator. The high selectivity exhibited by the etch process permits use of an electromagnetically coupled plasma generator, which in turn permits the etch process to be performed at low pressures of between 1 and 30 milliTorr, resulting the etching of vertical sidewalls in the oxide layer.

RELATED APPLICATION

This application is a division of Ser. No. 07/826,310, filed Jan. 24,1992, now issued U.S. Pat. No. 6,171,974 which is a continuation-in-partof Ser. No. 07/772,340 filed Jun. 27, 1991, abandoned.

FIELD OF THE INVENTION

This invention relates to a process for plasma etching oxide with highselectivity to nonoxide silicon-containing layers in an integratedcircuit structure in either a capacitive discharge orelectromagnetically coupled type of plasma reactor.

BACKGROUND ART

Oxide layers typically are utilized as insulation, overlying silicon orsilicon-containing surfaces, e.g., single crystal silicon such as asilicon wafer, epitaxial silicon, polysilicon, or silicides such astitanium silicide in integrated circuit structures. Such as oxide layersmay be selectively etched, for example, to form vias for formation ofconductive contacts to the underlying silicon. Coventionally such oxideetches are carried out in a plasma etch process utilizing one or morefluorine-containing etch gases such as, for example, CF₄, CHF₃, CH₂F₂,CH₃F, C₂F₆, NF₃, SF₆, etc.

When conventional capacitive discharge plasma generators are utilized insuch prior art oxide etching processes, the pressure in the etch chamberis typically maintained at about 100 to 1000 milli-Torr (1 Torr),resulting in a selectivity, with respect to silicon, of about 20:1. Thatis, oxide is preferentially etched instead of silicon by a ratio ofabout 20:1.

However, the use of such a high pressure during the etch adverselyaffects control of the etch profile. For example, to achieve verticalwalls in 0.35 micrometer (μm) diameter contacts and/or vias, lowerpressures of below 200 milliTorr, preferably below 30 milliTorr, andtypically about 10 milliTorr, must be used. However, at such lowpressures, the use of a conventional parallel plate capacitive dischargetype plasma generator may result in a low etch rate and higherpeak-to-peak voltage, necessitating the use of another type of plasmagenerator such as an electromagnetically coupled type plasma generator.

While the use of a pressure of about 10 millliTorr and a plasmagenerated by an electromagnetically coupled plasma generator does resultin the etching of contact holes with vertical walls, the selectivity tosilicon of such an etch system, using the previously discussedfluorine-containing etch gas chemistry, is reduced to about 6:1,probably due to the difficultly of polymer formation in such a lowpressure environment and the more aggressive nature of the higherdensity plasma which results from the use of such an electromagneticallycoupled plasma generator instead of the capacitive discharge type plasmagenerator.

Such a low selectivity may by satisfactory for a highly planarizedstructure and for perfectly uniform etch/plasma chamber conditions.However, such a low selectivity is unacceptable in many applicationswhere it is highly desirable to etch as little silicon as possible oncesuch silicon is exposed during the etching of the overlying oxide. Forexample, it is desirable in some instances, to etch less than about 5 nm(5×10⁻³ μm) of underlying silicon during the oxide etch.

Such conventional plasma oxide etch processes, using one or moreconventional fluorine-containing etch gases, usually rely on theformation of a polymer to inhibit etching of the silicon, whereinliberation of oxygen, during etching of the oxide, breaks down thepolymer on the oxide surface, while the absence of such generated oxygenprevents breakdown of the polymer over silicon surfaces. It is,therefore, necessary to increase such polymer generation to achieve ahigher selectively than the above mentioned 6:1 oxide to silicon etchratio for electromagnetically coupled type plasmas.

However, such an increase in polymer formation increases the selectivityof the process at the expense of a reduction of the oxide etch rate, areduction of the process window, and an increase in the chances forparticle formation. This, in turn, can result in an unacceptablereduction in through-put of the process and a reduction of device yield.

It would, therefore, be desirable to provide an oxide etch process whichwould exhibit high selectivity to silicon, even when used at lowpressure, e.g., about 10 milliTorr, with an electromagnetically coupledplasma generator, without any substantial reduction in etch rate of theoxide material.

SUMMARY OF THE INVENTION

The oxide etch process of the invention comprises the plasma etching ofoxide over a silicon-containing surface using a mixture of SiF₄ gas andone or more fluorine-containing etchant gases to provide a processhaving high selectivity with respect to the silicon-containing surface.Preferably the etch chamber in which the process is carried out alsocontains an exposed silicon surface.

In a preferred embodiment, the etch process of the invention is carriedout at a pressure of from about 1 to about 30 milliTorr, typically about10 milliTorr, using a plasma generated by an electromagnetically coupledplasma generator. The etch process may, however, be used at higherpressures of from about 50 to about 200 milliTorr, typically about 100milliTorr, using a plasma which may be generated by either theabove-mentioned electromagnetically coupled plasma generator or by acapacitive discharge (parallel plate) type plasma generator.

The oxide etch process of the invention exhibits high selectivity tosilicon of as much as 30:1, i.e., oxide is etched at a rate as much as30 times the etch rate of silicon, regardless of the type of plasmagenerator utilized, or the pressure utilized within the broad range offrom about 1 to about 200 milliTorr.

BRIEF DESCRIPTION OF THE DRAWING

The sole drawing is a flowsheet illustrating a process of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The oxide etch process of the invention comprises a highly selectiveplasma etch for the plasma etching of oxide over a non-oxidesilicon-containing surface on an integrated circuit structure in an etchchamber using a mixture of SiF₄ gas and one or more fluorine-containingetchant gases. Preferably the etch chamber also contains an exposedsilicon surface.

The one or more fluorine-containing etchant gases used in the process ofthe invention in combination with SiF₄ will, of course, be understood tomean a fluorine-containing etchant gas (or gases) other than SiF₄. Suchfluorine-containing etchant gases may comprise one or more 1-2 carbonfluorine-containing hydrocarbon gases such as, for example, CF₄, CHF₃,CH₂F₂, CH₃F, C₂F₆, and mixtures of the same. Other fluorine-containingetchant gases such as NF₃, SF₆, and mixtures of same may also be used,as well as mixtures of such fluorine-containing etchant gases with 1-2carbon fluorine-containing hydrocarbon etching gases.

The one or more fluorine-containing etchant gases used in combinationwith SiF₄ in the practice of the process of the invention may alsocomprise one or more higher molecular weight fluorinated hydrocarbons.Higher weight fluorinated hydrocarbons are defined as 3-6 carbonfluorinated hydrocarbon compounds having the general formulaC_(x)H_(y)F_(z), wherein x is 3 to 6, y is 0 to 3, and z is 2x−y (forcyclic compounds) or 2x−y+2 (for non-cyclic compounds). Such 3-6 carbonfluorinated hydrocarbons comprise an organic molecule: containing eithercarbon and fluorine (hydrogen-free fluorocarbon); or carbon, fluorine,and hydrogen (hydrofluorocarbon); and which may be either cyclic ornon-cyclic, but not aromatic.

Examples of cyclic 3-6 carbon fluorinated hydrocarbon compounds whichmay be included in the above formula are: C₃H₃F₃, C₃H₂F₄, C₃HF₅, C₃F₆,C₄H₃F₅, C₄H₂F₆, C₄HF₇, C₄F₈, C₅H₃F₇, C₅H₂F₈, C₅HF₉, C₅F₁₀, C₆H₃F₉,C₆H₂F₁₀, C₄HF₁₁, and C₆F₁₂. Examples of non-cyclic 3-6 carbonfluorinated hydrocarbons compounds which may be included in the aboveformula are: C₃H₃F₅, C₃H₂F₆, C₃HF₇, C₃F₈, C₄H₃F₇, C₄H₂F₈, C₄HF₉, C₄F₁₀,C₅H₃F₉, C₅H₂F₁₀, C₅HF₁₁, C₅F₁₂, C₆H₃F₁₁, C₆H₂F₁₂, C₄HF₁₃, and C₆F₁₄.Preferred among the above 3-6 carbon fluorinated hydrocarbon compoundsis cyclooctofluorobutane (C₄F₈).

Any of these higher weight fluorinated hydrocarbon etchant gases may beused alone or in combination with any of the other previously discussedfluorine-containing etchant gases, together with SiF₄.

The amount of SiF₄ gas used in the etch chamber should range from about10 to about 50 volume percent of the total amount of fluorine-containingetchant gas (or gases) used. Thus, for example, when one or morefluorine-containing etchant gases are flowed into a 9 liter etch chamberat a flow rate from about 20 standard cubic centimeters per minute(sccm) to about 60 sccm, the flow rate of SiF₄ will range from about 2sccm (10 volume % of 20 sccm) to about 30 sccm (50 volume % of 60 sccm).When a larger or smaller etch chamber is used, the flow rates may needto be respectively adjusted either upwardly or downwardly, but the ratioof SiF₄ gas to the total of the one or more fluorine-containing etchantgases used in the process will remain the same.

The mixture of SiF₄ and one or more fluorine-containing etch gases maybe used alone in the etch chamber or may be further diluted using one ormore inert gases such as helium or argon. Such inert gases may be flowedinto the etch chamber at a rate of 0 to about 200 sccm. In someinstances, nitrogen or other non-reactive gas or gases may also be usedwith the mixture of SiF₄ and one or more fluorine-containing etch gases(with or without inert gases).

The plasma etch process of the invention using a combination of SiF₄ andone or more fluorine-containing etch gases (with or without other gases)maybe used in combination with a conventional capacitive discharge(parallel plate) plasma generator or with an electromagnetically coupledplasma generator. The plasma associated with the etch chamber during theetch process of the invention may comprise a plasma generated within theetch chamber or generated external to the etch chamber itself, but inthe etch gas flow upstream of the chamber.

Furthermore, as more fully described in U.S. Pat. No. 5,556,501, theparent of which was filed on Jun. 27, 1991 by us and others, assigned tothe assignee of this invention, and cross-reference to which is herebymade, an electrode, having a silicon-containing surface, may also beprovided in the etch chamber. This silicon-containing electrode may beoptionally maintained at the rf bias. The presence of thissilicon-containing electrode in the etch chamber during the etch hasbeen found to be beneficial with or without an RF bias on the electrode.However, if an RF bias is not applied to the electrode, it is beneficialto maintain the silicon-containing surface of the electrode at anelevated temperature of from about 200° C. to about 300° C. to inhibitpolymer deposition on the surface.

While we do not wish to be bound by any theories of operation, it isbelieved that the presence of the silicon-containing electrode in theetch chamber during the etch process inhibits the presence of excessamounts of free fluorine radicals in the chamber, i.e., acts as abuffer.

The pressure used during the etch process of the invention may vary fromas little as 1 milliTorr to as high as 200 milliTorr. It will be noted,however, that it may not be possible to use a pressure below about 50milliTorr when using a capacitive plate type plasma generator because ofthe inability of such a plasma generator to ignite or sustain a plasmain a pressure below about 50 milliTorr.

Therefore, preferably the pressure is maintained within a range of fromabout 50 milliTorr to about 200 milliTorr when using a capacitivedischarge type plasma generator in the process of the invention.However, since it is highly desirable to operate the process of theinvention below a pressure of about 50 milliTorr, for example, toachieve vertical wall openings in the oxide layer such as vias, aspreviously discussed, preferably the process the invention is practicedutilizing a plasma generator which is capable of operating in thebroader pressure range of from about 1 milliTorr to about 200 milliTorr,and most preferably in the pressure range of from about 1 milliTorr toabout 30 milliTorr. An electromagnetically coupled plasma generator is,therefore, advantageously used in the practice of the process of theinvention.

The term “electromagnetically coupled plasma generator” is intended todefine any type of plasma generator which uses an electromagnetic field,rather than a capacitively coupled generator, to generate a plasma. Suchelectromagnetically coupled plasma generators can generate a plasmahaving an ion density of greater than about 10¹⁰ ions per cubiccentimeter which is characterized herein as a “high density” plasma,which is the preferred plasma density for use in the process of theinvention.

Included within the term “electromagnetcially coupled plasma generator,”for example, is an electron cyclotron resonance (ECR) type plasmagenerator such as described in Matsuo et al. U.S. Pat. No. 4,401,054;Matsuo et al. U.S. Pat. No. 4,492,620; and Ghanbari U.S. Pat. No.4,778,561 (cross-reference to which three patents is hereby made); aswell as in an article by Machida et al. entitled “SiO₂ PlanarizationTechnology With Biasing and Electron Cyclotron Resonance PlasmaDeposition for Submicron Interconnections,” published in the Journal ofVacuum Science Technology B, Vol. 4, No. 4, July/August 1986, at pp.818-821.

Also included in the term “electromagnetically coupled plasma generator”for example, is an inductively coupled helical or cylindrical resonatorsuch as described in Steinberg et al. U.S. Pat. No. 4,368,092 or Flammet al. U.S. Pat. No. 4,918,031, cross reference to both of which patentsis hereby made.

Further included in the term “electromagnetically coupled plasmagenerator” for example, is a helicon diffusion resonator such as theplasma generator described in Boswell U.S. Pat. No. 4,810,935,cross-reference to which is also made.

Ogle U.S. Pat. No. 4,948,458, cross-reference to which is also herebymade, describes yet a further type of electromagnetically coupled plasmagenerator comprising a transformer coupled plasma generator.

The power level of the plasma may vary from about 500 watts to about 5kilowatts (kW), depending upon the particular type of plasma generator,size of chamber, desired etch rate, etc. For example, using an ECR typeelectromagnetically coupled plasma generator in association with a etchchamber of about 6 liters and a desired etch rate of about 500 nm perminute, the power would typically range from about 2 to about 3 kW. Foran inductive type electromagnetically coupled plasma generator used inassociation with a 2 liter etch chamber and a desired etch rate of about500 nm per minute, the power would typically range from about 1 to about2 kW. When a high density plasma is to be generated, the power density,i.e., the power level relative to the volume of the plasma generatingchamber, should be equivalent to a power level of about 1000 watts in a4 liter plasma generating chamber.

A further advantage of the process of the invention is that the highselectivity to silicon exhibited by the use of silicon tetrafluoride(SiF₄) permits the use of a “leaner” fluorine-containing etch gas orgases in combination with the SiF₄ gas which will generate less polymerand thus permit a faster etch rate. By a “leaner” fluorine-containingetch gas which will generate less polymer is meant a fluorine-containingetch gas which does not contain any carbon, such as NF₃ or SF₆; orcarbon and fluorine-containing gas having a fluorine to carbon ratio(F/C) of at least 3:1, e.g., such as CHF₃, and preferably at least 4:1,e.g., CF₄.

To further illustrate the practice of the invention, a 6 inch diametersilicon wafer, having a layer of oxide thereon, and a patternedphotoresist mask formed over the oxide layer, was placed in a 9 literplasma etching chamber maintained at a pressure of about 10 milliTorr. Amixture of about 12 sccm of SiF₄ and 30 sccm of CF₄ was flowed throughthe chamber while a plasma was ignited and maintained at a power levelof about 2500 watts, using an electromagnetically coupled plasmagenerator.

The oxide etch was carried out for about 60 seconds, after which theplasma was extinguished, the flow of gases shut off, and the etchedwafer removed from the etch chamber. The etched oxide layer wasexamined, using SEM, and contact holes, having an average diameter ofabout 0.35 microns and vertical walls, with respect to the surface ofthe oxide layer, were found to have been formed in the oxide layer.

Thus, the process of the invention provides an improved plasma etchprocess for oxide wherein the use of a combination of silicontetrafluoride and one or more fluorine-containing etch gases provides anetch system having a high selectivity for silicon. This high selectivitypermits the optional use of leaner fluorine-containing etch gases, e.g.,gases having low carbon to fluorine ratio, which will generate lesspolymer and thus provide a faster etch rate. The use of this mixture ofetch gases also permits use of either capacitive discharge orelectromagnetically coupled type plasma generators in the process. Theprocess may be carried out at pressures ranging from about 1 to about200 milliTorr. The process may be carried out using anelectromagnetically coupled plasma generator, preferably at a lowpressure, ranging from about 1 to about 30 milliTorr, typically about 10milliTorr; or may be carried out using a capacitive discharge typeplasma generator, preferably at a higher pressure of from about 50 to 25about 200 milliTorr, typically about 100 milliTorr. Use of anelectromagnetically coupled plasma generator permits the process to beused at a low pressure of from about 1 to 30 milliTorr which, in turn,permits formation of vertical sidewall openings in the oxide layer beingetched.

What is claimed is:
 1. A plasma etch process for a plasma processingchamber provided with an interior non-oxide silicon-containing surfaceand adapted to accept a substrate for processing, the processcomprising: flowing a fluorine-containing etching gas into the chamber;forming a plasma from the etching gas to etch the substrate; andmaintaining the non-oxide silicon-containing surface at a temperature ofat least 200° C.
 2. The process of claim 1, wherein temperature is nomore than 300° C.
 3. The process of claim 1, wherein said non-oxidesilicon-containing surface is a silicon surface.
 4. The process of claim1, wherein said plasma processing chamber is an electromagneticallycoupled plasma reactor.
 5. The process of claim 4, wherein saidelectromagnetically coupled plasma reactor is inductively coupled. 6.The process of claim 1, wherein said plasma processing chamber is acapacitive discharge plasma generator.
 7. The process of claim 1,wherein said plasma is formed by coupling power into said chamber usingelectron cyclotron resonance.
 8. The process of claim 1, wherein saidplasma is formed by coupling power into said chamber through a helicalresonator.
 9. The process of claim 1, wherein said fluorine-containingetching gas is a carbon-containing etching gas.
 10. The process of claim9, wherein said etching gas contains a gas consisting of fluorine andcarbon atoms.
 11. The process of claim 1, wherein said non-oxidesilicon-containing surface is incorporated into an electrode.
 12. Theprocess of claim 11, further comprising applying RF power to saidelectrode.
 13. The process of claim 1, further comprising maintaining anRF bias on said silicon-containing surface.
 14. The process of claim 1,wherein said flowing step flows a silicon-containing gas into saidchamber.
 15. A plasma etch process comprising: a) flowing afluorine-containing etching gas into a plasma etching chamber, saidchamber adapted to receive and support a substrate to be etched andfurther comprising a separate and exposed silicon-containing surfacewithin said chamber; b) forming a plasma from said etching gas includinginductively coupling power into said chamber so as to etch saidsubstrate positioned in said chamber; and c) maintaining said separatesilicon-containing surface at a temperature of at least 200° C., therebyinhibiting free fluorine radicals in said chamber.
 16. The process ofclaim 15, wherein said temperature is no more than 300° C.
 17. Theprocess of claim 15, wherein said silicon-containing surface is asilicon surface.
 18. The process of claim 15, wherein said forming stepuses a helical resonator to couple said power into said chamber.
 19. Theprocess of claim 15, wherein said fluorine-containing etching gascontains a fluorocarbon gas consisting of carbon and fluorine atoms. 20.The process of claim 19, wherein said fluorocarbon gas consists ofC_(x)F_(z), where x is 3 to
 6. 21. The process of claim 15, wherein saidfluorine-containing etching gas does contain an effective amount ofelemental hydrogen.
 22. The process of claim 10, wherein saidfluorocarbon gas consists of C_(x)F_(z), where x is 3 to
 6. 23. A plasmaetch process for a plasma processing chamber provided with an interiornon-oxide silicon-containing surface and adapted to accept a substratefor processing, the process comprising: flowing a fluorine-containingetching gas into the chamber; etching the substrate with a plasma formedfrom the etching gas; and maintaining the non-oxide silicon-containingsurface at a temperature of at least 200° C.